A fuel cell is an electrochemical cell which generates electricity by combination of hydrogen and oxygen. Unlike a general chemical cell such as a dry cell or storage cell, the fuel cell can generate electricity continuously as long as the necessary hydrogen and oxygen are supplied. In addition, the fuel cell has no heat loss so that efficiency of the fuel cell is twice as high as efficiency of internal combustion engine. Furthermore, since the fuel cell directly converts chemical energy generated by combination of hydrogen and oxygen into electric energy, the fuel cell is eco-friendly, and is capable of being operated without worries about the exhaustion of fossil fuel.
Depending on the type of electrolyte, the fuel cell may be classified into polymer electrolyte fuel cell, phosphoric acid fuel cell, molten carbonate fuel cell, solid oxide fuel cell, and alkaline fuel cell.
One of the most important factors to improve performance of the polymer electrolyte fuel cell is to maintain moisture content by supplying predetermined moisture to polymer electrolyte membrane of membrane-electrode assembly. This is because the generating efficiency is rapidly deteriorated as the polymer electrolyte membrane gets dried.
A method for humidifying the polymer electrolyte membrane is a membrane humidifying method for supplying moisture to dry reaction gas by the use of polymer separation membrane.
The membrane humidifying method uses a membrane which selectively permeates only vapor contained in unreacted gas, to thereby supply the vapor contained in the unreacted gas to the polymer electrolyte membrane. This method is advantageous in that it can manufacture a small-sized humidifier with lightness in weight.
If the selective permeation membrane used for the membrane humidifying method forms a module, it is preferable to use hollow fiber membranes having a large permeation area per unit volume. That is, if fabricating the humidifier with the hollow fiber membranes, the hollow fiber membranes having a large contact surface area can be highly integrated so that the fuel cell is sufficiently humidified even with small volume. In this case, the humidifier with the hollow fiber membranes can be fabricated of a low-priced material. Also, moisture and heat contained in unreacted gas discharged at a high temperature from the fuel cell may be collected and reused in the humidifier.
FIGS. 10 and 11 illustrate a humidifier for fuel cell according to the related art. The humidifier for fuel cell according to the related art comprises a membrane housing 210 in which a plurality of bundles of hollow fiber membranes 270 are integrated to supply moisture to reaction gas flowing through the lumens of the hollow fiber membranes 270; a second inlet 221 for introducing high-humidity unreacted gas; and a second outlet (not shown) for discharging the unreacted gas.
However, in case of the humidifier for fuel cell according to the related art, the high-humidity unreacted gas introduced into the inside of the membrane housing 210 flows concentratedly toward the lowest-pressure region, that is, less-congested region of the hollow fiber membranes 270. Thus, the moisture is smoothly supplied only to the reaction gas flowing through the lumens of the hollow fiber membranes 270 which are brought into sufficient contact with the high-humidity unreacted gas. Meanwhile, the moisture is not supplied to the reaction gas flowing through the lumens of the hollow fiber membranes 270 which are not brought into contact with the high-humidity unreacted gas. Accordingly, the humidifying performance is deteriorated in the humidifier for fuel cell according to the related art. Also, it is difficult to bring the high-humidity unreacted gas into uniform contact with the respective hollow fiber membranes 270 due to the unequal distribution of the high-humidity unreacted gas, which might cause a large difference in contamination level among the hollow fiber membranes 270, whereby a replacement period of the hollow fiber membrane 270 becomes shortened.